DC/DC switching regulators or converters are part of many power management systems because of the improved power conversion efficiency provided by these regulators over that of linear regulators. Switching regulator designs can achieve efficiencies as high as 90% or more where as the efficiency of linear regulators is dependent strictly upon the difference between the input voltage and the output voltage. DC/DC switching regulators can take the form of a buck or voltage reducing regulator, a boost converter, which increases the input voltage, or a buck/boost converter, which can perform both functions.
It has now become common place for active loads to enter reduced power or “sleep” modes in which portions of the circuit not currently necessary for operation are shut off in order to conserve power. These power management techniques allow battery power devices to run for much longer periods time on the same charge. They are also useful in devices powered from the power mains as a way of reducing both the energy consumed by the device and the heat generated thereby. When DC/DC converters are operating at their normal load current, the average current through the inductor is continuous in order to supply this current to the load. However, should the current fall below a threshold value, the current through the inductor will go to zero during some portion of the converter cycle. This is known in the art as “discontinuous mode” operation because the current through the inductor is no longer continuous. At this point, it is generally pointless to operate the DC/DC converter as the amount of power needed to operate the converter greatly exceeds that consumed by the load. The converter may be shut down at this time and the load supplied by the output capacitor until the voltage falls below a predetermined threshold value or a predetermined amount of time has elapsed.
Circuits for detecting the discontinuous operation of a DC/DC converter are known in the art. For example, in the U.S. Pat. No. 5,481,178, a circuit for controlling a switching regulator is shown in an inductor designed to have a current tap or a series resistor is utilized to detect the current flow in the converter. When the current falls below a predetermined threshold, the system is placed in a low power mode. One disadvantage to this technique is that the inductor must be especially manufactured to have the current tap formed therein. Thus standard devices can not be utilized. In the embodiment in which a resistor is utilized, the power dissipated in the resistor reduces the efficiency of the regulator. Both techniques suffer from the disadvantage that additional pins are required on the integrated circuit containing the rest of the regulator circuitry which increases the cost of the integrated circuit.
It is also known in the art to measure the voltage across a synchronous rectifier in a DC/DC converter and turn the synchronous rectifier OFF if reverse current flow is detected. Measuring the voltage across the synchronous detector transistor requires a more expensive comparator having a low offset unless the ON resistance of the synchronous rectifier is made high enough to generate a voltage that exceeds the offset of the comparator. The former technique increases the cost whereas the latter technique decreases the efficiency of the converter.